This invention relates to a low thermal expansion ceramic product and to a process for its preparation.
Extensive efforts have been made to develop and manufacture ceramic products having flameware type bodies. Included among these products are those that can withstand the thermal shock which is associated with successive exposure of formed products to normal household oven and freezer temperatures. Such products must necessarily be fabricated from materials which have a suitable plasticity to facilitate shaping for a variety of household uses and must also have a high resistance to thermal shock. Few products of this type have been commercially successful either because of the cost of the starting materials or because of costs involved in the manufacturing process. Some commercially available low expansion products are made by focusing a composition into glass, pouring the glass into a mold and shaping a crystalline product by crystal nucleation and growth under very controlled thermal conditions. The disadvantage of these types of manufacturing processes are that they require expensive manufacturing facilities.
Suitable products having coefficients of linear expansion of less than 2.5.times.10.sup.-6 .degree. C. have also been prepared from various combinations of clay with various types of lithium aluminum silicates using standard pottery making materials, equipment and processes. A typical process of this type is described in U.S. Pat. No. 3,279,930 issued to Roland R. Van Der Beck, Jr. on Oct. 18, 1966. The starting materials used in the process include a kaolin or ball clay and a relatively pure ceramic grade of .alpha. spodumene. The materials are sintered at a suitable temperature and time (between Orton cones 12 and 16) to provide a composition containing 14% to 36% by weight of mullite and 64% to 86% by weight of .beta. spodumene. Deviations from the specified process result in solid solutions which fall outside of the desired ranges and which do not have expansion coefficients which are less than 2.5.times.10.sup.-6 .degree. C. The process has several disadvantages including the high cost and lack of availability of relatively pure ceramic grade spodumene. Another disadvantage involves the process conditions which must be carefully controlled to ensure that the final .beta. spodumene solid solution and mullite composition contains mullite within a specific range to provide a suitable low thermal expansion coefficient. Needless to say, such carefully controlled conditions effect the cost of the final product.
The coefficients of linear expansion of .beta. spodumene and mullite are 0.5.times.10.sup.-6 .degree. C. and 4.5.times.10.sup.-6 .degree. C. respectively. The expansion coefficients for various compositions of the two materials generally are a linear function of the amount of each of the materials in the final composition. It is recognized that other materials may be present in ceramic products such as those disclosed by Van Der Beck, and that such materials can effect the coefficient of expansion of the product. However, this effect is somewhat insignificant when the final product is comprised primarily of .beta. spodumene solid solution and mullite. Thus, it is advantageous to minimize the formation of mullite in order for the final product to have an expansion coefficient which is as close as possible to that of .beta. spodumene. Invariably however, some mullite will be formed during the process of converting .alpha. spodumene to .beta. spodumene because of the starting materials used and time-temperature conditions utilized to effect the conversion.
Based on the foregoing discussion it is evident that a need exists for a low-thermal expansion ceramic which may be economically produced from inexpensive materials and is useful for flameware and household purposes.